NORTHERN latitudes got off lightly this winter, even though the ozone layer over the Arctic suffered a record thinning. Up to 30 per cent of the ozone layer beyond 60掳 North disappeared in February and March. But 鈥渋f the timing had been different, it could have been a lot worse鈥�, says Neil Harris of the European Ozone Research Coordinating Unit in Cambridge.
Luckily, the two physical ingredients required to do maximum damage in the atmosphere 鈥� extreme cold and sunlight returning after the long Arctic night 鈥� did not coincide. The Arctic was especially cold in December, but there was no sun.
鈥淚f the same temperatures had occurred in February as they sometimes do, there would have been much more ozone loss,鈥� says Harris.
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The risk of a catastrophic ozone loss, of the kind seen over Antarctica each spring, is likely to increase over the next five years 鈥� and perhaps much longer suggests Harris. As a result, people in the northern hemisphere could be exposed to powerful ultraviolet radiation.
The stratospheric concentration of chlorine, the main chemical that destroys the ozone, will increase by about 10 per cent over the next five years, before beginning to fall, says Harris. This is despite the introduction in the 1980s of tough controls on emissions of ozone-eating chemicals. Chlorine compounds released in the 1980s are still floating up towards the stratosphere.
But even after peak chlorine concentrations have passed, other factors may continue to put the ozone layer at risk, he says. This is because the ice clouds within which most ozone destruction occurs are becoming more frequent over the Arctic. There are two reasons for this. 鈥淔irst, the stratosphere is becoming colder, and secondly it contains more water vapour,鈥� says Harris.
The cooling is partly the result of the thinning ozone layer itself, and partly due to the greenhouse effect, which causes warming at lower levels in the atmosphere but cooling in the stratosphere. The additional water vapour is probably produced by the breakdown of methane in the atmosphere which comes from the guts of farm animals, rice paddies, landfills, coal mines and gas pipelines.
鈥淭he more water vapour there is up there, the warmer the temperatures at which the clouds can form,鈥� says Harris. 鈥淪o the conditions for ozone destruction will occur more often and over wider areas.鈥�